1. Field of the Invention
This invention relates generally to the molding art and more particularly to an improved method of and means for molding fiber reinforced composite tubes.
2. Prior Art
The prior art is replete with a vast assortment of techniques and equipment for molding plastics and other materials into tubular shapes and other similar shapes. Examples of such molding techniques and equipment are described in the following patents:
Re.20,460 1,457,986 3,107,158 1,142,342 1,949,650 3,128,322 1,146,413 2,342,988 3,377,657 1,177,240 2,406,843 3,520,961 1,424,386 2,999,780 1,455,039 3,015,855
The present invention is concerned with molding tubes of fiber reinforced composite materials. As is well known to those versed in the art, a fiber reinforced composite is a composite material consisting of a mass of reinforcing fibers bonded together into a rigid shape by a resin. In some cases the reinforcing fibers have a random orientation. In other cases, the reinforcing fibers may have a parallel orientation or may be arranged in several layers each having a parallel fiber orientation, with the fibers in the adjacent layers oriented in different directions to provide the fiber reinforced composite with selected mechanical and thermal expansion properties.
Such fiber reinforced composites may comprise various reinforcing fibers and various resins. The present invention is concerned primarily with molding graphite fiber reinforced epoxy tubes. It will become readily evident as the description proceeds, however, that the invention may be utilized with other fiber reinforced composites.
Simply stated, molding a fiber reinforced composite tube involves application of an annular, liquid-resin-impregnated layer of reinforcing fibers to a cylindrical mold or mandrel and curing of the resin under heat and pressure. The curing pressure expells entrapped air and excess resin from the fiber-resin layer. The curing heat converts the liquid resin to a solid resin which bonds the reinforcing fibers into a rigid mass having a tubular shape corresonding to the mandrel diameter.
The reinforcing fibers, such as graphite fibers, which are utilized in fiber reinforced composites are commercially available in strips and mats which may be impregnated with resin and wrapped around a mandrel to form the annular fiber-resin-layer on the mandrel. As noted above, the fibers may have a parallel or random orientation and may be applied to the mandrel in several layers each having a parallel fiber orientation, with the fibers in the adjacent layers oriented in different directions to attain selected mechanical and thermal expansion properties of the finished composite tube.
Molding fiber reinforced composite tubes in this manner presents a problem to which this invention is addressed. This problem resides in the fact that during the curing process, the reinforcing fibers tend to shift circumferentially of the mandrel and thereby produce wrinkles in the finished tube. As is well known by those versed in the art, such shifting of the fibers and resulting wrinkling of the finished tube may be avoided by radially compacting the fibers prior to curing.
According to one current method of compacting the fibers, a heat shrinkable tube is slid over the fiber-resin layer on the molding mandrel and is heated to shrink or constrict the tube about the layer. This contraction of the tube causes the latter to exert a radial compression force on the fiber-resin layer which compacts the fibers sufficiently to prevent their circumferential shifting during subsequent curing. After this compaction of the fibers has been accomplished, the assembly consisting of the mandrel, heat shrink tubing, and the compressed intervening fiber-resin layer is sealed in a plastic bag or bladder which is evacuated to produce an additional radial pressure on the layer and placed in an oven or autoclave to cure the resin and thereby form a rigid fiber reinforced composite tube.
While this prior method of molding a fiber reinforced composite tube produces a satisfactory finished product, it has three major limitations. One of these limitations resides in the fact that heat shrinkable tubing is available only in certain sizes which dictate the composite tube diameters which may be molded. The second limitation of the current molding procedure is that the heat shrink tubing can be used only once and then must be discarded. Considering the relatively high cost of this tubing, which is on the order of $16.00 to 60.00 per foot above 6-inch tubing diameters, it is apparent that the current molding procedure is relatively costly and adds substantially to the cost of the finished product. The third limitation of the current procedure is that the latter is limited to cylindrical tube shapes and to molding the composite about an inner mandrel, such that the surface of the finished tube which conforms to the mandrel or mold is the inner surface of the tube. Other methods of composite tube fabrication involve the use of tapered telescoping molds which are quite costly and lend themselves only to the fabrication of straight tubes, and a female mold with an internal bladder which results in a rough tube surface.